Fuel cells for combining hydrogen and oxygen to produce electricity are well known. A known class of fuel cells includes a solid-oxide electrolyte layer through which oxygen anions migrate; such fuel cells are referred to in the art as “solid-oxide” fuel cells (SOFCs).
In some applications, for example, as an auxiliary power unit (APU) for a transportation application or a stationary power unit (SPU) for a stationary application, an SOFC is preferably fueled by “reformate” gas, which is the effluent from a catalytic liquid or gaseous hydrocarbon oxidizing reformer, also referred to herein as “fuel gas”. Reformate typically includes amounts of carbon monoxide (CO) as fuel in addition to molecular hydrogen. The reforming operation and the fuel cell operation may be considered as first and second oxidative steps of the hydrocarbon fuel, resulting ultimately in water and carbon dioxide. Both reactions are preferably carried out at relatively high temperatures, for example, in the range of 700° C. to 1000° C.
A complete fuel cell stack assembly includes a plurality of fuel cells, for example, 60 cells in the form of sub-assemblies, and a plurality of components known in the art as interconnects which electrically connect the individual fuel cell subassemblies in series electrically. Typically, the interconnects include a conductive foam or weave disposed in the fuel gas and air flow spaces adjacent the anodes and cathodes of the fuel cells.
In known prior art fuel cell stack assemblies, each subassembly contains porting which, when joined to the porting of adjacent subassemblies, creates a supply manifold and an exhaust manifold for both the cathode air to the plurality of cathodes and the fuel gas to the plurality of anodes. Thus, all of the cathodes are in parallel pneumatic flow and all of the anodes are in parallel pneumatic flow. The total air is divided among the plurality of cathodes such that each increment of air passes over only a single cathode and then is collected in the air exhaust manifold. Similarly, the total fuel gas entering the stack assembly is divided among the plurality of anodes such that each increment of fuel gas passes over only a single anode and is then collected in the fuel gas exhaust manifold. This flow scheme is sensitive to uneven flow distribution at low anode flow rates. Generally, only a portion of the fuel cell gas is consumed, or utilized, in the single pass through the stack. High fuel utilization is desirable for high system efficiency; however, stack power density decreases with increasing fuel utilization due to fuel gas concentration gradients in the SOFC stack.
Air entering a prior art SOFC stack assembly at ambient temperature must be pre-heated to accommodate and regulate the temperature of the SOFC stack; and to this end, it is known to pass the incoming air through a cathode air heat exchanger immediately ahead of the fuel cells using hot exhaust air as the heat source, thus increasing the thermal efficiency of the system (recuperation). Also, the fuel gas is typically formed in a hydrocarbon reformer and thus may exit the reformer at about 650° C., although both the fuel gas and the cathode air are desirably substantially hotter than 650° C. for optimum fuel consumption and electrical generation (stack efficiency).
Another area of concern in prior art fuel cell stack assemblies is the temperature rise through the stack, the hydrogen/oxygen reaction being highly exothermic. High temperature gradients produce high stresses within the stack and can reduce stack durability. Temperature gradients through the traditional stack may be reduced through increased cathode air massflow, but this results in reduced air utilization for the fuel cell system and a resulting loss of system efficiency.
What is needed in the art is a means for increasing the fuel efficiency of the fuel cell system and electrical output of an SOFC stack assembly.
What is further needed in the art is a means for decreasing thermal stresses within a stack assembly, thereby improving stack durability, without reducing system air utilization and decreasing system efficiency.
It is a principal object of the present invention to increase the electrical output of an SOFC multi-cell stack of a given size.
It is a further object of the present invention to increase the fuel efficiency of an SOFC stack module.
It is a still further object of the present invention to extend the working lifetime of an SOFC stack assembly.
It is a still further object of the present invention to reduce thermal stresses in the SOFC stack, without reducing system air utilization, or for a given thermal stress on the SOFC stack, increase system utilization.